1. Field of the Invention
The present invention generally relates to a thermal head drive circuit and a printer using the same, and particularly relates to a thermal head drive circuit that drives plural thermal heads according to print data in a parallel form and a printer using the same.
2. Description of the Related Art
As one way of performing printing on thermal paper by melting the dye, linearly-arranged thermal heads are used that generate heat upon application of electric current from corresponding drive circuits.
FIG. 8 is a circuit diagram of a related-art thermal head drive circuit 1. With reference to FIG. 8, the thermal head drive circuit 1 is a semiconductor IC that includes a serial/parallel conversion unit 2, a delay latch unit 3, and an output unit 4.
Print data are input in serial fashion from an input terminal 5 to the serial/parallel conversion unit 2. The serial/parallel conversion unit 2 converts the print data according to a clock CLK, which is input from an input terminal 6, to output the print data in an n-bit parallel fashion. The output bits of the print data are supplied to corresponding AND circuits 71-72N of the delay latch unit 3.
A latch signal with a value of “1” is input from an input terminal 8 to the delay latch unit 3 in synchronization with the completion of the serial-parallel conversion of the print data. The latch signal with the value 1 is sequentially delayed by serially-connected delay elements 91-92N-1. The latch signal from the input terminal 8 and the delayed latch signals from the delay elements 91-92N-1 are supplied to the AND circuits 71-72N of the delay latch unit 3, respectively. In response to the latch signals with the value 1 supplied from the input terminal 8 and the delay elements 91-92N-1, the AND circuits 71-72N latch the corresponding bits of the print data and supply the latched bits of the print data to output FETs (field-effect transistors) 101-102N of the output unit 4, respectively.
The output FETs 101-102N have sources connected to ground and drains connected to output terminals 111-112N, respectively. Thermal heads 121-122N are connected at first ends to the output terminals 111-112N, respectively, as loads, and at the other ends to a power source 13. The thermal heads 121-122N are aligned in a line on a thermal head substrate 14.
When the output FETs 101-102N are turned ON according to the corresponding bits of the print data, electric current is applied to the thermal heads 121-122N corresponding to the turned-ON output FETs 101-102N. Thus, the thermal heads 121-122N generate heat to perform printing on thermal paper.
Japanese Patent Laid-Open Publication No. 2000-246938 discloses a recording head driving device that includes plural groups of thermal head driving circuits. A delay circuit for delaying a strobe signal is disposed upstream of each of a second and subsequent groups of the driving circuits.
In a printer, plural pairs of the thermal head drive circuit 1 and the thermal head substrate 14 of FIG. 8 are disposed adjacent to one another such that the thermal heads of the thermal head substrates 14 are aligned in a line.
In this printer, because each thermal head drive circuit 1 outputs bits of the print data in the order from the output terminal 111 to the output terminal 112N, heat is transferred across the corresponding thermal head substrate 14 from the thermal head 121 that first generates heat toward the thermal head 122N that last generates heat. This results in a temperature distribution as shown in FIG. 9 in the thermal head substrates 14 of the printer, in which the temperature gradient is discontinuous at the boundaries between the adjacent thermal head substrates 14, causing color irregularities in the printed result.